Why is the nominal capacity of power plants given in terms of power and not in terms of energy?

What do you mean by "in terms of energy"? Power = energy per second so why do you think power isn't "in terms of energy"?

I suppose you could specify the energy output in Joules over the life time of the power station but is that very useful on a day to day basis? If the load needs 10MW at 9am on Monday then you need to know if the power station can deliver that much power. It's no good knowing that over it's 40 year lifetime it can deliver 12 * 1015 Joules